1. Field of the Invention
The present invention relates to methods of inspection usable, for example, in the manufacture of devices by lithographic techniques and to methods of manufacturing devices using lithographic techniques.
2. Description of the Related Art
A lithographic apparatus is a machine that applies a desired pattern onto a substrate, usually onto a target portion of the substrate. A lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In that instance, a patterning device, which is alternatively referred to as a mask or a reticle, may be used to generate a circuit pattern to be formed on an individual layer of the IC. This pattern can be transferred onto a target portion (e.g. comprising part of, one, or several dies) on a substrate (e.g. a silicon wafer). Transfer of the pattern is typically via imaging onto a layer of radiation-sensitive material (resist) provided on the substrate. In general, a single substrate will contain a network of adjacent target portions that are successively patterned. Known lithographic apparatus include steppers, in which each target portion is irradiated by exposing an entire pattern onto the target portion at one time, and scanners, in which each target portion is irradiated by scanning the pattern through a radiation beam in a given direction (the “scanning” direction) while synchronously scanning the substrate parallel or anti-parallel to this direction. It is also possible to transfer the pattern from the patterning device to the substrate by imprinting the pattern onto the substrate.
In order to determine features of the substrate, such as its alignment, a beam is reflected off the surface of the substrate, for example at an alignment target, and an image is created on a camera of the reflected beam. By comparing the properties of the beam before and after it has been reflected off the substrate, the properties of the substrate can be determined. This can be done, for example, by comparing the reflected beam with data stored in a library of known measurements associated with known substrate properties.
Such a system of illuminating a target and collecting data from the reflected radiation is often used to illuminate a plurality of superimposed patterns, for example gratings. The second pattern has a predetermined bias compared to the first pattern. By analysing the characteristics of the reflected radiation it is possible to determine the overlay error OV, between the patterns.
Analyzing the characteristics of the higher diffraction orders of the reflected radiation can be used to determine small overlay errors, for example of the order of ±20 nm. Similarly, errors of the order of the pitch of the grating can be detected using a method described in U.S. application Ser. No. 11/455,942. However, overlay errors which are smaller than the pitch (usually 400 nm-1 μm) but larger than the bias (usually 5-20 nm) between the gratings can sometimes go undetected. An example of this is shown in FIG. 4 in which the top left hand figure shows illumination with TM polarized light (solid line) and TE polarized light (dashed lines). In this example there is a predetermined bias between the gratings of ±15 nm. The asymmetry resulting from a first pair of overlayed gratings, shown by open circles, indicates a (correct) overlay error of 0. However, the asymmetry resulting from a second pair of overlayed gratings (shown by solid dots) also indicates an overlay error of 0 which is incorrect as the correct overlay error is 70 nm.
Such overlay errors which are greater than the bias but smaller than the pitch of the grating can therefore sometimes go undetected.